This application claims the benefit of United Kingdom Application No. 0029121.1 filed Nov. 29, 2000.
This invention relates, in general, to a mechanism and apparatus for coherently recovering and interpreting data from an optical disc, such as encoded on a digital versatile disc read only memory (DVD-ROM) or the like. More particularly, but not exclusively, the present invention provides an apparatus and method that supports a mirror averaging function to generate, in the context of data recovery and through the implementation of a ground referred capacitor, a mirror signal from optical data on an optical disc.
With respect to the storage of data on optical storage media, such as on compact disc read only memory (CD-ROM) and DVD-RAM, a selected form of modulation encodes data into the surface of the media. In the context of DVD-ROM or DVD-RAM, an eight-fourteen modulation (EFM) scheme is used to encode binary data through data xe2x80x9cpitsxe2x80x9d that are either magnetically or optically inscribed within, or manually embossed/stamped on, the surface of the optical storage medium and undisturbed mirror regions. The length of the pit or complementary mirror is indicative of the encoded binary information, subject to there being no defects associated with the formation of the pit or mirror.
The data segments (or sectors) spiral outward from a center of the optical storage medium. The data segments are also indexed through a header that is embossed (e.g., physically stamped) onto the surface of the optical storage medium. The header providing address and location information, such as track and sector numbers. The headers are individually indexed at the beginning of the disc for scanning. The headers have a precisely defined width dimension and are separated by a data sector of defined length.
From a perspective of data recovery, once on-track, an array of (typically) four photodiodes recovers the information stored on the medium. The four photodiodes provide an output current that varies according to an amount of reflectivity from the surface of the medium. More particularly, laser light is reflected from the marks and spaces, with a data pit (i.e., a mark) providing an inferior reflectivity and hence a lower current than a space that provides maximum reflectivity and hence maximum current. Essentially, with respect to data pits, destructive interference at the photodiode is generally indicative of EFM modulation. The array is conventionally in the form of a 4-quadrant photodiode array in which a sum of the individual currents from the photodiodes is used to extract EFM data. Specifically, the four currents from the photodiode are converted to voltages in a pre-amplifier of a pick-up assembly before being presented to a read channel. The variation in the reflected laser light levels results in voltage signals of varying magnitude and duration, (i.e., read frequency) the data, which are processed by the read channel to produce raw binary data.
When reading data from CD and DVD ROM discs, it is necessary to detect regions of the disc that contain defects. The regions take two forms (i) areas where the reflectivity of the disc is greatly reduced (i.e., defects) and (ii) areas where the data region is at maximum reflectivity (i.e., interrupts). Detection of the regions is necessary in order to perform certain holding and corrective functions in the player/recorder to maintain the data channel (principally the read channel) in a stable state during such defective regions. For example, at detection of a defect, the phase lock loop and/or the tracking servo can be frozen. Failure to detect a defect otherwise results in the loss of coherent data in that particular region of defect, thus requiring a full recovery. Once the device is on-track, the device should be held on-track to avoid having to instigate a full recovery action for coherent data recovery.
Furthermore, the ability to resolve mirror (i.e., high reflectivity) regions is important in the context of device operation since mirror regions inherently exist between adjacent tracks of an optical disc. Consequently, monitoring of the photodiode output during a track seek or jump mode where a read/write head (or pick-up assembly) moves radially across multiple contiguous tracks results in a sinusoidal modulation of the envelope of the read frequency data (mirror signal or mirror modulation). Such mirror modulation is used to extract a mirror component to control radial servo movement since the sinusoidal oscillation can be used to determine radial distance and relative movement between tracks by counting periods in the mirror signal. In contrast with read frequency on-track data (which has a frequency of several megahertz), mirror modulation during track transition has a relatively low frequency of about 100 kilohertz.
In terms of the mirror modulation, while the oscillation (i.e., the variation or perturbation) of the read frequency envelope edge is referred to as being xe2x80x9csinusoidalxe2x80x9d, it will be understood that the mirror modulation may take other forms and is generally more triangular in its nature. Consequently, the term xe2x80x9csinusoidalxe2x80x9d should not be considered as limiting but merely descriptive of how mirror modulation affects the read frequency envelope 34.
In summary, the mirror modulation is extracted from the composite read frequency (RE) signal to produce what is commonly referred to as the read frequency reference pointer (RFRP) signal. The RFRP signal has to be appropriately biased and its voltage swing optimized such that the RFRP signal can be sliced effectively to generate the mirror signal during a track seek operation. The RFRP signal may be used to be positioned in a range of an associated digital-to-analog converter (DAC) that processes the RFRP signal for servo control. To date, rebiasing and repositioning have been accomplished in read channel schemes by coupling the composite read frequency signal externally to the integrated circuit (IC) into the mirror amplifier. Such a configuration requires two pins and a coupling capacitor. Unfortunately, the mechanism of coupling is considered sub-optimum, since AC coupling produces a high pass filter characteristic in the circuit. Moreover, the high pass filter characteristic causes level variations in the upper and lower thresholds of the mirror modulation of the RFRP signal during seek. Indeed, the level variations significantly complicate the extraction (and slicing) of the mirror. If AC coupling is adopted in the context of a mirror recovery circuit, the AC coupling results in drift in the mirror frequency that is caused by an RC time constant of the effective high pass filter. Consequently, there is a requirement to constantly adjust top and bottom hold circuits to maintain a correct slice level for accurate mirror detection and track identification purposes.
Methods for detecting low reflectivity defect regions are well documented and readily available in CD and DVD read channel integrated circuits. It will be understood that defects, in general, have an affect of corrupting a mirror modulation signal, with the defect causing de-focusing of the laser at the photodiode to a dark level, whereas interrupts generate spurious mirror signals. The defect takes the recovered signal components below a read frequency envelope/threshold associated with a data pit.
According to a first aspect of the present invention there is provided a mirror circuit for an optical disc media device, the mirror circuit comprising an amplifier. The amplifier comprises a first input, a second input and an output. The amplifier may be arranged to receive an averaged DC level of a read frequency envelope derived from modulated data on an optical disc. The first input may be coupled to a first potential divider network that is further responsive to a biasing signal having a rebias level. The second input may be responsive to a signal having a mirror modulation component and a data component substantially corresponding to the averaged DC level. The output may provide an amplified signal output. The output may be coupled to the second input through a feedback path. The feedback path may form part of a second potential divider network matched to the first potential divider network. The biasing signal may cause a level swing induced by the mirror modulation component in the amplified output signal to be below the rebias level. The rebias level may appear as the amplified output signal when the data component is present in the signal applied to the second input.
Typically, the first potential divider network includes a first resistor coupled to the first input and a first variable resistor coupled to the first input and further coupled in parallel with the first resistor. The first variable resistor may be arranged to receive the biasing signal. The second potential divider network generally includes a second resistor coupled to the second input and a second variable resistor within the feedback path and coupled to the second input.
An averaging circuit may also be provided. The averaging circuit may be coupled to the first input. The averaging circuit may preferably contains an in-line resistor coupled to receive the signal and a ground-referred capacitor coupled to ground between the in-line resistor and the first input. Alternatively, a digital averaging circuit is coupled to receive the signal and arranged to provide the averaged DC level to the first input.
In a second aspect of the present invention there is provided a method of biasing an extracted mirror component from an optical disc, the extracted mirror component used for slicing to generate a mirror signal, the method comprising: (A) applying to a first input of a differential amplifier an averaged DC level of a read frequency envelope derived from modulated data on the optical disc; (B) applying to a second input of the differential amplifier a signal having a mirror modulation component and a data component substantially corresponding to the averaged DC level; and (C) applying a biasing signal having a rebias level to the first input. The biasing signal may cause a level swing induced by the mirror modulation component in an amplified output signal to be below the rebias level and further to cause the rebias level to appear as the amplified output signal when the data component is present in the signal applied to the second input.
The averaged DC level is preferably generated through one of: holding the DC component of the signal on a ground-referred capacitor electrically isolated from the first input; and generating a digital voltage representation of the averaged DC level. The method may further comprise: applying the averaged DC level to the first input through a first potential divider network responsive to the biasing signal; and providing a feedback path for the amplified output signal to the second input. The feedback path forms part of a second potential divider network matched to the first potential divider network.
In a further aspect of the present invention there is provided a method of biasing an extracted mirror component from an optical disc supporting data in a signal envelope, the extracted mirror component derivable from mirror modulation of the signal envelope, the method comprising: (A) filtering the signal envelope to generate, over time, a first signal having: (i) a DC offset component representative of the data and (ii) mirror modulation providing variations in level from the DC offset component; and (B) rebiasing the first signal to produce an intermediate signal in which level swings attributable to the mirror modulation are below a reference level associated with the DC offset component.
The rebiasing may include applying a biasing signal having a rebias level to a non-inverting input of a differential amplifier. The biasing signal may cause the level swings in an amplified output signal to be below the rebias level and may further cause the rebias level to appear as the amplified output signal when data is present in the signal applied to an inverting input of the differential amplifier. In one example, the method may further comprise slicing the intermediate signal to generate pulses of a mirror signal.
A particular embodiment further comprises generating a high passed version of the first signal as the amplified output signal, the high passed version generated in a feedback path to the inverting input. The feedback path may have a variable gain and the method may further include maintaining correspondence between the gain in the feedback path and a gain associated with the biasing signal.
The ground referred capacitor scheme of the preferred embodiment is used in the mirror averager circuit to derive the on-track level for the composite read (SUMDC_INT) signal. By holding the composite read signal during seek and track jump modes of operation, an upper limit of the RFRP signal can be maintained at a predetermined level. One embodiment of the present invention advantageously provides a robust mirror recovery mechanism that is less likely to suffer from inaccurate mirror frequency determination, since the mirror modulation is relatively stable and the RFRP signal is dynamically rebiased to optimize slicing.
Exemplary implementations of the present invention allow the analog RFRP signal (containing a representation of the mirror) to be reliably sliced to generate a square wave mirror signal. The mirror may be generated by defining a suitable slice threshold (using peak and trough hold functions) between peak-to-peak variations in the RFRP signal. The RFRP signal is positioned where the top and bottom hold can operate with maximum resolution.